1. Field of the Invention
This invention relates to a CCD (Charge Coupled Device) area image sensor and its operating method, and more particularly to a structure of the CCD area image sensor for being driven in both of line sequential scanning and interlace scanning and a method for driving it in a line sequential scanning.
2. Description of the Related Art
Since a required band width for processing a video signal sensed by the interlace scanning is a half of a video signal sensed by the line sequential scanning, the interlace scanning has been widely used in the television standard systems of various nations, the industrial television systems and so on. However, the interlace scanning causes a considerable deterioration of reproduced picture such as an interline flicker noise. Due to the deterioration, new television systems are planned to use the line sequential scanning to improve the definition of the reproduced picture.
As for the imaging devices, the development of the CCD area image sensors is outstanding. Those CCD image sensors obtainable in the market are all designed for the interlace scanning. The CCD area image sensors for the line sequential scanning cannot be obtained in the present market. There are two main reasons for this market condition. First, the CCD area image sensors are manufactured to be used in home-use video cameras which is only one market having a large demand. Second, it is technically difficult to form CCD vertical shift register having a very small electrode pitch required in the CCD area image sensors for the line sequential scanning. The electrode pitch in the image sensors for the line sequential scanning is a half of the image sensors for the interlace scanning, because, although transfer gate electrodes of one stage in the vertical shift register of the image sensor for the line sequential scanning are arranged for each sensing element, those of the image sensor for the interlace scanning correspond to each two sensing elements.
Furthermore, as above-mentioned, the CCD image sensors for the line sequential scanning and the interlace scanning have different structures. The scanning method is limited in accordance with the structure of the CCD image sensors. The CCD image sensor usable in both scanning systems is not devised.
One measurement for using one image sensor in both scanning systems may be to change sequence of output signals from an area image sensor by an external circuit. That is, output signals in two fields are derived from an area image sensor by the interlace scanning to be stored in a memory, and then the output signals of each picture elements in the two horizontal lines are alternatively read out from the memory.
However, since the output signal in one field is photo-charges accumulated in sensing elements for two field periods prior to the output operation, the photo-charge accumulation periods for the output signals in two fields which are to be synthesized overlap one field period, resulted in a deteriorated dynamic resolution (i.e. a resolution of moving picture). Moreover, a large-scale external circuit is required for synthesizing the output signals in two field. Especially, there need an expensive field memory, synthesizing switching means, many line memories and other circuits for driving them. Therefore, the external circuit is very expensive and has a very complicated structure.